In the context of the present invention the term “hearing device” refers to hearing aids (alternatively called hearing instruments or hearing prostheses) used to compensate hearing impairments of hard of hearing persons as well as to audio and communication devices used to provide sound signals to persons with normal hearing capability, e.g. in order to improve hearing in harsh acoustic surroundings. Such hearing devices are miniature ear-level devices which typically are employed for extended periods of time and are powered by small battery cells such as a zinc air battery or increasingly by rechargeable batteries such as for instance a Nickel Metal Hydride (NiMH) accumulator. The power consumption of such a hearing device is preferably monitored in order to provide the user with timely notice that the battery needs to be replaced or recharged, i.e. by means of an “end of battery life” indicator. It is therefore an important requirement to have a reliable means by which the battery charge can be determined. This is typically done by voltage monitoring and level comparison. The component that drains the battery the most in such hearing devices is the receiver, i.e. the miniature loudspeaker that outputs sound waves to be perceived by the user of the hearing device. Hence, information regarding the current consumption of the receiver is a good basis for establishing the state of charge of the battery.
Hearing device batteries can be modelled as an ideal voltage source generating an open circuit voltage VOC in series with an internal resistance or battery impedance Rint. These are internal battery parameters which cannot be measured directly. However, the battery voltage VBat across the battery terminals and the battery current IBat provided by the battery are observable parameters. The internal parameters are linked to the observable ones via the linear relation VBat=VOC−Rint·IBat. Linear regression can thus be used to determine estimates of VOC and Rint, corresponding to the intercept point and the slope of the trace VBat(IBat), when the observable parameters VBat and IBat are known. The battery voltage VBat can be measured directly and the battery current IBat can be determined by inserting a shunt resistor between the battery and the load and measuring the voltage drop across the shunt resistor. In order to measure the receiver current a series resistor could be inserted either on the supply line of the class D power amplifier driving the receiver or in the branches of the power amplifier. However, adding a series resistor on the supply impacts the maximum power output (MPO) of the hearing device. In fact, the overall impedance on the supply up to the receiver inputs and including the power amplifier output should be minimised in order to support high power hearing devices.